(a) Field of the Invention
The present invention relates to a switch control device for constantly controlling a maximum output power of a switching mode power supply (hereinafter referred to as “SMPS”).
(b) Description of the Related Art
Output efficiency of an SMPS varies depending on an input voltage. Assuming that the SMPS supplies a constant output power to a load, as the input voltage of the SMPS is reduced, a larger input current is required such that a duty of a switch of the SMPS increases. As a result, conduction loss of the switch increases, such that the output efficiency of the SMPS decreases. In addition, in general, a maximum value of a drain current of the SMPS is set to a predetermined threshold value or less regardless of the input voltage.
In this case, a maximum output power of the SMPS varies depending on the input voltage. The output power is determined depending on the drain current, input voltage, and output efficiency of the SMPS. In an overload state, the switch is driven at a maximum duty, such that a maximum value of the drain current is maintained at a threshold value. Therefore, as the input voltage is increased, the output power increases. That is, the maximum output power of the SMPS varies depending on the input voltage. As a result, when the SMPS is subjected to a high input voltage and an overload, the SMPS is under severe stress.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.